This invention is useful in infusing a porous material with a gas, which may be comprised of a volatile substance or suspended solids, and removing a gas from a porous substance. A significant application of the invention is in soil vapor extraction (SVE), which is also referred to as soil venting, air stripping, vacuum extraction, etc. The invention will be explained primarily in terms of this application, but it is to be understood that there are numerous other applications for use of the inventive method. For example, the invention may be used in drying gypsum wallboard material and bioremediation, where microbes are used to destroy contaminants in soil or other porous materials and oxygen must be added to maintain the microbe population.
SVE is often the preferred method to clean up soil which is contaminated with volatile materials. In numerous locations, chemicals such as gasoline and other hydrocarbons have leaked into the earth or have been dumped into the earth. These chemicals, often referred to as volatile organic compounds (VOC's), migrate down through the earth and contaminate groundwater, imperiling water used by much of the population. It is generally agreed that it is preferable to remove the contaminants before they reach the water table. In its basic form, SVE involves drilling one or more wells, installing solid well casing above the contaminated soil and porous well casing where a well passes through the contaminated soil, and using a vacuum source to reduce the pressure in the well or wells. The vacuum source is usually an industrial blower having its suction side connected to the well by piping. VOC's contaminating the soil are drawn out of the soil as a result of reduced pressure in the well and are passed into means for capturing or destroying them, such as activated carbon, on which they are absorbed. VOC's in the soil in liquid form are vaporized. Air from the atmosphere usually passes through the contaminated soil into the well and carries VOC's to the vacuum source. Air may be blown into an air supply well having a porous casing, using a blower or other pressure source, to increase the flow of air which sweeps VOC's from the soil. A steady flow or an intermittent flow of gas is used. The present invention provides methods in which oscillatory pressures and flows are used to enhance the extraction rate of volatile substances.
Since information on how to conduct SVE can be easily found, it is not necessary to discuss it in detail herein. U.S Pat. No. 4,593,760 (Visser et al.) teaches a basic method of SVE. U.S. Pat. No. 4,730,672 (Payne) teaches an SVE process in which a vacuum source is used to remove air and VOC's from a borehole, the VOC's are separated from the air, and the air is recycled back into the ground by means of a pressure source to pick up more VOC's; the present invention can be employed in the practice of this process by applying oscillation to both the flow of gas into the ground and the flow out of the soil or to just one of the flows. The use of both injection wells and extraction wells is also discussed in U.S. Pat. No. 5,032,042 (Schuring et al.). U.S. Pat. No. 5,067,852 (Plunkett) teaches a process for removing contaminants from excavated material; the present invention may be used with this process. The present invention may also be used with the process of U.S. Pat. No. 4,842,448 (Koerner et al.), which involves placing an impermeable barrier over a contaminated area and reducing the pressure under the barrier to draw out VOC's. U.S. Pat. No. 4,895,085 (Chips) and 4,983,364 (Buck et al.) deal with separating VOC's from an air stream pulled from an extraction well.